In recent years, different types of cellular networks for wireless communication have been developed to provide radio access for various wireless devices in different areas. The cellular networks are constantly improved to provide better coverage and capacity to meet the demands from subscribers using services and increasingly advanced terminals, e.g. smartphones and tablets, which may require considerable amounts of bandwidth and resources for data transport over a radio interface in the networks. As a result, it is common to configure a cellular network with cells of varying types and sizes, e.g. in an overlapping fashion, to provide needed capacity and flexibility depending on expected traffic intensity in different areas, the cells thus forming a so-called heterogeneous cellular network.
In this disclosure, the term “User Equipment, UE” will be used to represent any user-controlled wireless communication device or terminal capable of radio communication including transmitting uplink signals and receiving downlink signals transmitted from a radio node over a transmission point. Thus, the term UE could for example be exchanged for wireless communication device throughout this disclosure. Further, the term “transmission point” is commonly used in this field and represents any antenna equipment, typically including antenna element, tower and radio head, from which downlink radio signals can be transmitted to UEs and also used for reception of uplink signals from the UEs. Throughout this description, a transmission point could thus also be referred to as a “transmission and reception point” but the term transmission point will be used for short. The radio nodes described here may include so-called high power nodes, commonly also referred to as macro nodes, and low power nodes, each node controlling one or more transmission points to serve different connected UEs.
A heterogeneous cellular network may comprise hierarchically arranged nodes, including macro nodes transmitting with relatively high power and covering relatively large areas of a size in the order of kilometers, and low power nodes transmitting with relatively low power and covering areas of a size in the order of a few meters, e.g. micro, pico, femto and relay nodes, to mention some customary examples. The low power nodes may be employed together with the macro nodes in an overlapping fashion to locally provide added capacity in so-called “hot spot” areas such that multiple small areas served by such micro/pico/femto/relay nodes may be located within the larger area served by a macro node.
A heterogeneous network may be realized basically in two different ways, according to currently existing solutions. In a first alternative, a macro node and multiple low power nodes cover individual cells with different cell identities, where multiple small cells are overlapped by a large macro cell served by the macro node. In such a network with separate cells being served by respective transmission points, a UE is served by a single transmission point at a time and must undergo handover between the cells when necessary to maintain a radio connection.
In a second alternative, the macro node and the low power nodes cover a single common or “combined” cell and all nodes therein use a single shared cell identity, which means that a UE in the cell can basically be served by several transmission points at the same time. The cell served by more than one transmission points of such a cell, is usually called a “cluster cell” or “Coordinated Multiple Point, CoMP, cluster” or CoMP cell. A CoMP cluster with shared cell identity may even be called a shared cell or cell with shared or common cell identity. The term cluster cell will be used throughout this disclosure. CoMP may also be used in a scenario where a macro node and multiple low power nodes within the coverage of the macro node use individual cell identities.
In one example, the transmission points using a common cell identity may also be regarded as a distributed radio node or base station with multiple antennas at different locations in the cell. In another example, a plurality of transmission points comprising at least one high power node and multiple low power nodes may be combined to serve a cell. The transmission points in a cluster cell may use the same cell ID, or cluster cell ID, or the transmission points may use different individual cell IDs. In this type of network, one or more transmission points may be controlled by a radio node serving the cluster cell. For example, each transmission point may be controlled by its own radio node located at the transmission point, or several transmission points may be controlled by a common radio node, and so forth.
The latter alternative of using a cluster cell with multiple transmission points and corresponding radio nodes has the advantage of eliminating the need for performing handover when moving from one transmission point to another which reduces the amount of signaling and also reduces the risk of dropped connection due to failed handover, among other things. Uplink radio signals, e.g. containing data, sent from the UE are received by multiple transmission points and corresponding radio nodes which may be able to decode and process the signals jointly. On the other hand, any downlink radio signals directed to the UE may be sent from multiple transmission points of the cluster cell which may enhance reception and decoding of the signals.
In FIG. 1, a cluster cell is served by a macro transmission point 100 covering basically the whole area of the cluster cell, and a plurality of low power transmission points 102, 104, 106, 108 and 110 each covering a limited part of the whole cluster cell area. A UE located somewhere in the cluster cell may be connected to all or some of the transmission points 100-110 at the same time such that these transmission points receive uplink radio signals sent from the UE. Further, the UE may receive and decode radio signals transmitted from at least some of the transmission points 100-110.
The UE may also perform measurements on the received radio signals and report these measurements to the network, e.g. over one or more of the transmission points, which network is then able to use the reported measurements for various purposes, e.g. for scheduling of radio resources, link adaptation, selection of antenna transmission mode, and so forth. The measured radio signals may comprise Channel State Information Reference Signals, CSI-RS, which are regularly transmitted from the transmission points. Such measurements by the UE are also often used for coordinating radio communication between neighboring cluster cells, such that the measurement results from a first cluster cell are communicated to and used by a neighboring second cluster cell to schedule transmissions therein in a way that reduces interference in the first cluster cell and/or in the second cluster cell. This is generally referred to as Inter-Cell Interference Coordination, ICIC. For example, an aggressor cluster cell may decide or agree to mute its transmissions during time intervals when interference-sensitive reference signals are transmitted in a neighboring victim cluster cell.
In order to make the above-described radio measurements efficient and useful, a UE-specific measurement set including a subset of all the transmission points of a cluster cell may be configured for a specific UE, wherein the UE is instructed to measure reference signals, such as CSI-RS, being transmitted from the transmission points of the UE-specific measurement set. Thereby, the transmission points providing the “best” signal strength and/or quality for the UE may hopefully be selected for inclusion in the UE-specific measurement set. In networks and systems based on Long Term Evolution, LTE, this is sometimes called the “COMP measurement set”. Typically, the transmission points being close to the UE and/or providing higher received power at the UE, in this example transmission points 102, 104 and 106, should be included in the UE-specific measurement set, as illustrated by the arrows therefrom in FIG. 1. The figure shows two example transmission points 108 and 110 which are not included in the UE-specific measurement set.
However, the selection of transmission points in the UE-specific measurement set is sometimes less than optimal such that some transmission points included in the measurement set do not provide satisfactory measurement results, e.g. due to insufficient or inaccurate signal strength and/or quality, while other transmission points not included in the measurement set might have provided better results and thus more useful measurements by the UE. It is thus a problem that one or more transmission points in the UE-specific measurement set sometimes cannot provide for, or enable, efficient and useful measurements of transmitted radio signals, and that these measurements may be a poor or even misleading basis for inter-cell coordination or more specifically inter-cluster cell coordination. This problem may reduce capacity and efficiency in resource usage in the cluster cell and its neighboring cluster cells. Another problem is that the employment of a cluster cell with shared cell identity may be suitable and beneficial in some situations but not in others.